Silicon carbide produced by the Acheson process is generally used to prepare sinterable silicon carbide powders, for economic reasons and because of its readier availability. Silicon carbide produced by the Acheson process, because of the high manufacturing temperatures required in the process, consists mainly of the thermodynamically more stable .alpha.-modification. Alpha-silicon carbide is used herein as meaning hexagonal and rhombohedral poly-types as well as a proportion of cubic SiC which cannot be excluded. Silicon carbide powders are obtained in the required fineness by intensive grinding of compact grains obtained from industrial SiC production. After grinding, the fine powders must subsequently be subjected to a wet chemical treatment in order to remove foreign substances resulting from grinding, especially dust from attrition of the grinding medium, adherent silica and free silicon which may interfere with the sintering process and/or lead to an undesirable increase in the residual oxygen or metal impurities in the finished sintered body. The dust from attrition of the grinding medium is customarily removed by a hydrochloric acid treatment and silica by a hydrofluoric acid and/or nitric acid treatment (cf. U.S. Pat. No. 4,230,497). Also known for the processing of SiC abrasive grain mixtures is a chemical treatment with hot lye (cf. "Ullmann's Enzyklopadie der technischen Chemie", vol. 21 (1982), page 435).
If free carbon must be removed, an annealing treatment of several hours duration at temperatures of at least 700.degree. C. can be used (cf. DE-A-29 12 443 and W. Bocker et al. in "Powder Metallurgy Int.", vol. 13 (1981), pages 37-39).
Sinterable boron carbide powders are generally produced from coarse-grained, crystalline boron carbide that has been manufactured on an industrial scale, by the melt reduction process, from boric acid and carbon in an electric furnace. The coarse grained boron carbide must first be subjected to intensive grinding to produce a powder of the required particle size and then it is subjected to a chemical treatment to remove undesirable impurities. Sinterable mixtures of silicon carbide and boron carbide can also be produced by grinding a mixture of silicon carbide and boron carbide and then subjecting the ground mixture to chemical treatment to remove sinter-impeding impurities (cf. DE-A-32 18 052).
These known processing methods for the manufacture of sinterable powders have several disadvantages which are exemplified as follows with reference to SiC:
1. The intensive grinding of the abrasive material which is customarily carried out by wet grinding in mills charged with a grinding medium, requires long grinding times to provide the required particle size. The long grinding time and abrasiveness of the material produces considerable wear on the grinding apparatus with a concomitant quantity of metallic impurities in the material. The long grinding time causes a distinct impairment in the quality of the SiC powder. Further, because of tribonchemical reactions which occur, the content of free carbon and silica in the powder is increased.
2. The undesirably high carbon content must be removed by subsequent annealing. The annealing disproportionally affects the smaller particle size portion of the SiC and oxidises it to SiO.sub.2.
3. In order to remove the large quantity of metallic impurities introduced by wear of the grinding apparatus, an intensive hydrochloric acid treatment is necessary which disproportionately affects the fine grain portion of the SiC.
4. The large amount of SiO.sub.2 produced during the processing must be removed by chemical purification, customarily by an environmentally harmful hydrofluoric acid treatment which generally does not reduce the amount of Si to a desirable level.
These known powder processing methods are, therefore, not only expensive in terms of time and expense but also involve losses in quality and yield.
The solution of the problem is to provide a process for the manufacture of sinterable silicon carbide and/or boron carbide powders having a maximum particle size of 1 .mu.m and finer, by wet grinding an aqueous suspension of the coarse grained material in mills charged with a grinding medium and subsequent chemical treatment, in which process the duration of grinding can be shortened and the purification can be carried out in a simpler and more effective manner without undue reduction in quality and yield of the powders.